Hi Richard et al.
The render setings were 13 hours, quality 0.8, AA 3, GI 1|1 - atmospheres had 75 samples, clouds ~150, fog ~100 samples.
I agree with the warp - though it is a ridged fractal that blends the clouds' density fractal. I wanted to get a bit more definition and overtuned ,-)
The ground was made with the new 'intersect underlying' feature. A took a while to get the appropiate settings (You should try to set the distribution to terrain-normal).
Thanks for your input,
Volker